This invention relates to aqueous quenching media and processes using the same for quenching metal substrates.
Various methods of heat treating metal substrates are known and include heating a metal substrate to an elevated temperature and then cooling. The cooling step, which is known in the art as “quenching”, typically is performed rapidly and is accomplished by immersing the hot metal substrate in a liquid quenching medium, i.e. a quenching bath, which typically is water or oil.
When the quenching medium is water alone, very rapid cooling of the metal substrate occurs. Rapid cooling is not suitable for many types of steel, since it tends to produce excessive strain which warps and cracks the steel. When the quenching medium is a hydrocarbon oil, a slower rate of cooling occurs. This can impart certain desirable physical properties in the metal substrate, including ductility in steel. Even though the slower cooling rate provided by oil quenching prevents or reduces excessive strain in the metal substrate, it often has the undesirable side-effect of preventing the metal substrate from adequately hardening.
A variety of aqueous media is available for quenching metal substrates and may include one or more of a polymer. For example, U.S. Pat. No. 3,220,893 discusses a quenching medium containing an oxyalkylene polymer having oxyethylene and higher oxyalkylene groups which form a desirable covering over the metal substrate surface during quenching. The polymer layer that coats the metal permits relatively short quenching times, thereby resulting in minimum internal stress of the metal substrate, minimum distortion of the metal substrate, and imparts uniform hardenability of the metal substrate.
U.S. Pat. Nos. 3,902,929, 4,826,545, and RE 34119 discuss aqueous quenching media containing a polyvinylpyrrolidone and U.S. Pat. No. 4,087,290 discusses an aqueous quenching medium containing a water-soluble polyacrylate, such as a sodium polyacrylate, which forms a vapor blanket about the metal substrate during the quenching operation.
Typically, aqueous polymer-based quenching media contain large amounts of polymer, e.g., 10 to 15% by weight, and “drag out” occurs during quenching in which the polymer coating that initially forms around in the metal substrate is removed. When drag out occurs, the viscosity of the quenching medium changes due to presence of solid polymer, thereby requiring an additional step of washing the quenched metal substrate to remove any of the solid polymer present on the metal substrate.
What is needed in the art are quenching media which will cool a heated metal substrate at a rate similar to oil-based quenching media at a rate that is between oil and water, while achieving the greatest degree of hardness without warping or cracking the metal substrate.